1. Field of the Invention
This invention relates generally to laminated magnetic thin films for magnetic recording and more particularly to magnetic thin films having multiple de-coupled ferromagnetic layers.
2. Description of the Related Art
A typical head and disk system 100 illustrated in FIG. 1 may include a magnetic transducer 102 supported by a suspension 104 as it flies above the disk 106. The magnetic transducer 102, usually called a “read/write head” or “slider,” may include elements that perform the task of writing magnetic transitions (the write head 108) and reading the magnetic transitions (the read head 110). The electrical signals to and from the read and write heads 110, 108 travel along conductive paths (leads) 112 which are attached to or embedded in the suspension 104. The magnetic transducer 102 is positioned over points at varying radial distances from the center of the disk 106 to read and write circular tracks (not shown). The disk 106 is attached to a spindle 114 that is driven by a spindle motor 116 to rotate the disk 106. The disk 106 comprises a substrate 118 on which a laminate 120 having multiple layers is deposited. The laminate 120 typically includes ferromagnetic layers in which the write head 108 records the magnetic transitions in which information is encoded.
Extremely small regions, or bits, on the ferromagnetic layers are selectively magnetized in chosen directions in order to store data on the disk 106. To increase the amount of data that can be stored on the disks 106 the number of bits per unit area, storage density, must be increased.
As the storage density of magnetic recording disks has increased, the product of the remanent magnetization Mr (the magnetic moment per unit volume of ferromagnetic material) and the magnetic layer thickness t has decreased. Similarly, the coercive field or coercivity (Hc) of the magnetic layer has increased. This has led to a decrease in the ratio Mrt/Hc. To achieve the reduction in Mrt, the thickness t of the magnetic layer can be reduced, but only to a limit because the layer will exhibit increasing magnetic decay, which has been attributed to thermal activation of small magnetic grains (the superparamagnetic effect). The thermal stability of a magnetic grain is to a large extent determined by KuV, where Ku is the magnetic anisotropy constant of the layer and V is the volume of the magnetic grain. As the layer thickness is decreased, V decreases. If the layer thickness is too thin, the stored magnetic information will no longer be stable at normal disk drive operating conditions.
One approach to the solution of this problem is to move to a higher anisotropy material (higher Ku). However, the increase in Ku is limited by the point where the coercivity Hc, which is approximately equal to Ku/Mr, becomes too great to be written by a conventional recording head. A similar approach is to reduce the Mr of the magnetic layer for a fixed layer thickness, but this is also limited by the coercivity that can be written. Another solution is to increase the intergranular exchange, so that the effective magnetic volume V of the magnetic grains is increased. However, this approach has been shown to be deleterious to the intrinsic signal-to-noise ratio (SNR) of the magnetic layer.
It is known that substantially improved SNR can be achieved by the use of a laminated magnetic layer of two (or more) separate magnetic layers that are spaced apart by a nonmagnetic spacer layer. This discovery was made by S. E. Lambert, et al., “Reduction of Media Noise in Thin Film Metal Media by Lamination”, IEEE Transactions on Magnetics, Vol. 26, No. 5, September 1990, pp. 2706-2709, and subsequently patented in IBM's U.S. Pat. No. 5,051,288. The reduction in intrinsic media noise by lamination is believed due to a decoupling of the magnetic interaction or exchange coupling between the magnetic layers in the laminate. The use of lamination for noise reduction has been extensively studied to find the favorable spacer layer materials, including Cr, CrV, Mo and Ru, and spacer thicknesses, from 5 to 400 angstrom, that result in the best decoupling of the magnetic layers, and thus the lowest media noise. This work has been reported in papers by E. S. Murdock, et al., “Noise Properties of Multilayered Co-Alloy Magnetic Recording Media”, IEEE Transactions on Magnetics, Vol. 26, No. 5, September 1990, pp. 2700-2705; A. Murayama, et al., “Interlayer Exchange Coupling in Co/Cr/Co Double-Layered Recording Films Studied by Spin-Wave Brillouin Scattering”, IEEE Transactions on Magnetics, Vol. 27, No. 6, November 1991, pp. 5064-5066; and S. E. Lambert, et al., “Laminated Media Noise for High Density Recording”, IEEE Transactions on Magnetics, Vol. 29, No. 1, January 1993, pp. 223-229. U.S. Pat. No. 5,462,796 and the related paper by E. Teng et al., “Flash Chromium Interlayer for High Performance Disks with Superior Noise and Coercivity Squareness”, IEEE Transactions on Magnetics, Vol. 29, No. 6, November 1993, pp. 3679-3681, describe a laminated low-noise disk that uses a discontinuous Cr film that is thick enough to reduce the exchange coupling between the two magnetic layers in the laminate but is so thin that the two magnetic layers are not physically separated.
Increased storage density while maintaining good thermal stability may be achieved by two ferromagnetic films antiferromagnetically coupled together across a nonferromagnetic spacer film. Some laminates may include two ferromagnetic films decoupled from one another and a third ferromagnetic film antiferromagnetically coupled to one of the ferromagnetic films. The third film is typically referred to as the antiferromagnetic slave layer. Because the magnetic moments of the two antiferromagnetically-coupled films are oriented antiparallel, the net remnant Mrt of the ferromagnetic layers is reduced by the Mrt of the antiferromagnetic slave layer. This reduction in Mrt is accomplished without a reduction in the thermal stability of the recording medium because the volumes of the grains in the antiferromagnetically-coupled films add constructively. The medium also enables much sharper magnetic transitions to be achieved with reduced demagnetization fields, resulting in a higher linear bit density for the medium.
In view of the foregoing it is clear that laminated magnetic thin films for magnetic recording must have a high signal-to-noise ratio. Accordingly, it would be advancement in the art to provide a laminated magnetic thin film with increased the signal-to-noise ratio compared to currently available media having multiple ferromagnetic layers with or without antiferromagnetically coupled layers.